Drive circuit for active matrix light emitting device

Details Drive circuit for active matrix light emitting device Drive circuit for active matrix light emitting device

2002-09-20

2003-12-30

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Plural power supplies

Plural cathode and/or anode load device

C345S211000, C345S055000, C345S082000, C345S092000

Reexamination Certificate

active

06670773

ABSTRACT:

This application is a continuation of International Application No. PCT/JP02/02592, filed Mar. 19, 2002, which claims the benefit of Japanese Patent Application No. 080504/2001,filed Mar. 21, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a drive circuit for a light emitting device for use in an image display apparatus, more particularly to a drive circuit for a light emitting device of active matrix type for driving a light-emitting device such as an organic or inorganic electroluminescent (hereinafter called “EL”)device or a light-emitting diode (hereinafter called “LED”), and a display panel of active matrix type utilizing such drive circuit.
2. Related Background Art
A display utilizing light-emitting devices such as organic or inorganic EL devices or LEDs arranged in an array and displaying a character by a dot matrix method is widely utilized in television, portable information terminal etc.
Such display based on the light-emitting device is currently interested because of the features, in comparison with the display utilizing liquid crystal, of the absence of a light source for illumination from the rear and a wider viewing angle. In particular, the display of so-called active matrix type, in which a static drive is executed by the combination of transistors and the aforementioned light-emitting devices, is currently attracting attention because of advantages of a higher luminance, a higher contrast and a higher definition, in comparison with the display of simple matrix drive based on time-shared drive.
Also for providing an image with gradation in the organic EL device, there can be conceived an analog gradation method, an area gradation method and a time gradation method as already known in the prior art:
(1) Analog Method
As an example of the conventional configuration, a simplest display device utilizing two thin film transistors (hereinafter called “TFT”)per pixel is shown in
FIGS. 6 and 7
. In
FIG. 6
, there are shown an organic EL element
101
, TFTs
102
,
103
, a scanning line
107
, a signal line
108
, a power supply line
109
, a ground potential
110
, and a memory capacity
111
utilizing a capacitor.
The circuit shown in
FIG. 6
functions in the following manner. When TFT
102
is turned on by the scanning line
107
, an image data voltage from the signal line
108
is accumulated in the memory capacity
111
. When the scanning line
107
is turned off to turn off TFT
102
, the above-mentioned voltage continues to be applied to the gate of TFT
103
whereby TFT
103
remains in the turned-on state.
On the other hand, the source electrode of TFT
103
is connected to the power supply line
109
, while the drain electrode is connected to one of the electrodes of the light-emitting element, and the gate electrode receives the image data voltage at the drain electrode of TFT
102
, whereby the current between the source electrode and the drain electrode is controlled by the above-mentioned image data voltage. The organic EL element, being connected between the power supply line
109
and the ground potential, emits light corresponding to the aforementioned current.
Since the amount of current depends on the gate potential of TFT
103
, the light emission intensity is regulated by changing the current characteristics in analog manner, utilizing an area (saturation area) where the source current as a function of the gate potential (Vg-Is characteristics) shows an upshift.
As a result, the light emission intensity of the organic EL element can be controlled and the display involving gradation can be realized. Such gradation representing method, utilizing an analog image data voltage, is called analog gradation method. In such method, the image data signal has to be adjusted in the gamma (&ggr;) characteristics according to the voltage-luminance characteristics of the organic EL element.
It is advantageous also for the light-emitting device, as in the liquid crystal display device or in the CRT, to enable gradational display by varying the light emission intensity of each pixel in order to achieve moving image display for the monitor of the personal computer or the television and also in order to ensure compatibility with the CRT. Also there will be obtained an advantage in cost, because of simplification in the driving system.
The aforementioned TFT currently includes that of amorphous silicon (a-Si) type and that of polycrystalline silicon (p-Si) type, but the latter is becoming more popularly employed because it shows a higher charge mobility enabling a finer configuration of the element and also because the progress in the laser working technology enables to execute the manufacturing process at a lower temperature. However, the polycrystalline silicon TFT is often influenced by the crystal grain boundary constituting the element, and tends to show a significant fluctuation from element to element in the Vg-Is current characteristics in the aforementioned saturation area. Therefore, such display device is-associated with a drawback of showing unevenness in the display, even if the video signal voltage entered into the pixels is uniform.
Also, the present TFT is mostly used as a switching element in an area where the drain voltage becomes constant as a function of the source voltage (such area being called a linear area) under the application of a gate potential considerably higher than the threshold voltage of the transistor, so that the aforementioned fluctuation in the saturation area is not much experienced.
(2) Area Gradation Method
On the other hand, an area gradation method is proposed in the reference AM-LCD2000, AM3-1. In this method, each pixel is divided into plural sub-pixels, each of which is on-off controlled to represent the gradation by the area of turned-on sub-pixels in the pixel.
In such method, the TFT can be utilized in the aforementioned linear area where the drain voltage becomes constant as a function of the source voltage, under the application of a gate potential much higher than the threshold voltage, so that the TFT can be used in a stable range of the characteristics and the light emission intensity of the light-emitting element is also stabilized. In such area gradation method, each element is on-off controlled and emits light at a constant intensity without gradational change, and the gradation is controlled by the area of the light-emitting sub pixels.
In this method, however, there can only be obtained digital gradation levels depending on the method of division of the sub pixels, and, in order to increase the number of gradation levels, it is required to increase the number of sub pixels with a reduction in the area thereof. However, even if the transistors are made smaller with the use of polycrystalline silicon TFTs, the area of the transistor portion in each pixel erodes the light-emitting area, thereby lowering the aperture rate of each pixel and reducing the light emission intensity of the display panel. Thus the luminance the gradational performance are in a trade-off relationship in which an increase in the aperture rate results in a decrease in the gradational performance, whereby it is difficult to improve the gradational performance.
(3) Time Gradation Method
In a time gradation method controls the gradation by the light-emitting time of an organic EL element, as reported in 2000SID36.4L.
FIG. 7
is a circuit diagram showing an example of a pixel portion of a conventional display panel employing the time gradation method. In
FIG. 7
there are shown an organic EL element
101
, TFTs
102
to
104
, a scanning line
107
, a signal line
108
, a power supply line
109
, a ground potential
110
, a memory capacity
111
and a reset line
112
.
In the time gradation method utilizing such circuit configuration, when TFT
103
is turned on, the organic EL element
101
emits light at the maximum intensity by the voltage from the signal line, while TFT
103
repeats on and off within a field time by TFT
104
and the gradation is represented by such light-emitting time.
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